In accordance with increase of interest in environmental issues and development of info-communication equipment, sensors for various gases have been developed. By grafting the sensors with semiconductor technology, it is easy to manufacture the sensors, and the manufactured sensors have improved performance. A primary goal of all sensors is to increase sensitivity for improving performance, and an effort to achieve this goal has also been increased.
Meanwhile, since a semiconductor gas sensor in the related art includes a semiconductor thin film as a sensing material, there is limitation in sensitivity. For example, it is almost impossible to sense stable chemical materials such as carbon dioxide (CO2).
Accordingly, in the sensor for sensing harmful gases such as carbon monoxide (CO), carbon dioxide, and the like, an electrochemical method using electrolyte solution, an optical method using infrared absorption, and a method for measuring electrical resistance of nano particles or nanowire have been applied.
The electrochemical method is to measure current flowing in external circuits caused by electrochemically oxidizing or reducing target gas, or to use electromotive force generated from ions in gas phase dissolved or ionized in an electrolyte solution or a solid, acting on an ionic electrode, which has disadvantages in that a reaction rate is extremely low, gas sensing range and environment for using the sensor are limited, and the cost is also high.
In addition, the optical method using infrared absorption is advantageous in sensing because it is rarely affected by other mixed gases or humidity; however, it has disadvantages such as complicated device, large dimension, and high cost.
Structures of chemical sensors are generally based on catalytic combustion type sensors, such that when the gas reacts with a platinum wire integrated in the sensor as a catalyst, the sensor is capable of sensing the gas by measuring change in resistance of the platinum wire resulted by endothermic reaction or exothermic reaction at the platinum wire, to thereby have improved stability and sensitivity of the sensor.
Meanwhile, as the relationship between contact reaction by chemical adsorption of a gas and electron density has been identified and an oxide semiconductor-type gas sensor has been developed and commercialized, the semiconductor-type gas sensor has been developed to be capable of sensing most gases including a combustible gas, which achieves miniaturization, cost reduction and improvement in reliability as compared to gas sensors based on other schemes.
As compared to other sensors that are required to be heated up to about 300° C. to detect nitrogen oxide, and the like, a gas sensor using carbon nanotube as one of the semiconductor-type gas sensor has advantages in that its sensitivity is thousands of times higher than those of other sensors because of its nanoscale size and it is able to be operated even at room temperature.
Gas sensors of measuring change in electrical resistance of a nanomaterial itself or a material coated on the nanomaterial according to concentration of a gas to be measured, has been developed. When the nanomaterials are used, the surface to volume ratio is significantly high, such that the effect of the surface reaction corresponding to the gas concentration change on the electrical resistance change in the volume is significantly large, thereby making it possible to manufacture a sensor having significantly high sensitivity.
Generally, in the existing sensors using nanoparticles or nanowires, electrical resistance is measured by connecting electrodes capable of measuring change in electrical resistance of nanomaterials only at specific portions of the nanomaterials dispersed non-uniformly on the substrate, or positioning the nanomaterials on the pre-patterned electrodes by flowing the nanomaterials or electrospinning the nanomaterials. These methods have disadvantages in that physical and electrical connections between the nanomaterials and the electrodes are unstable, and the nanomaterials being in contact with the substrate are affected by the substrate in a gas sensing process.
Afterward, a suspended nanowire-based sensor disclosed in Non-Patent Document 1 is manufactured by adhering a nanowire onto electrodes spaced apart from the surface at a predetermined gap, that is, attaching nanowires onto post-shaped electrodes using electrospinning, or selectively growing the nanowires from one electrode to another electrode in the opposite side. The existing suspended nanowire-based sensors have insufficient sensitivity, poor contact between the nanowire and the electrodes, difficulty in controlling manufacturing processes due to complexity of the processes, which causes reduction in yield, and require high fabrication cost and long process time. Therefore, the sensor has limitation in commercialization through mass-production of the sensor.